1. Field of the Invention
The invention relates to a process for producing granular polysilicon.
2. Description of the Related Art
Polycrystalline silicon, often also called polysilicon for short, is produced, for example, by means of the Siemens process. This involves heating thin filament rods of silicon in a bell jar-shaped reactor (“Siemens reactor”) by direct passage of current and introducing a reaction gas comprising a silicon-containing component and hydrogen. The filament rods are typically inserted vertically into electrodes present at the reactor base, through which they are connected to the power supply. Every two filament rods are coupled by means of a horizontal bridge (likewise made from silicon) and form a support body for the silicon deposition. The bridge coupling produces the typical U shape of the support bodies, also called thin rods. High-purity polysilicon is deposited on the heated rods and the bridge, as a result of which the rod diameter grows with time (CVD/gas phase deposition). After the deposition has ended, these polysilicon rods are typically processed further by means of mechanical processing to give chunks of different size classes, classified, optionally subjected to a wet-chemical cleaning operation and finally packed.
An alternative to the Siemens process involves fluidized bed processes in which granular polysilicon is produced. This is accomplished by fluidizing silicon particles by means of a gas flow in a fluidized bed, these being heated to high temperatures by means of a heating apparatus. Addition of a silicon-containing reaction gas results in a pyrolysis reaction at the hot particle surface. This deposits elemental silicon on the silicon particles, and the diameter of the individual particles grows. The regular removal of particles that have grown and addition of smaller silicon particles as seed particles allows the process to be operated continuously with all the associated advantages. Silicon-containing reactant gases that have been described are silicon-halogen compounds (e.g. chlorosilanes or bromosilanes), monosilane (SiH4), and mixtures of these gases with hydrogen.
While the polysilicon in the Siemens process is obtained in the form of a cylindrical silicon rod which has to be comminuted to chunks and possibly cleaned in a time-consuming and costly manner prior to the further processing thereof, granular polysilicon has bulk material properties and can be used directly as a raw material, for example, for single crystal production for the photovoltaics and electronics industry.
In the production of granular polysilicon in a fluidized bed reactor, it is necessary in the course of the process to meter silicon material into the reactor at regular intervals or continuously, and to withdraw ready-grown granular polysilicon from the reactor elsewhere.
US 2011024266 A1 discloses a method for conveying granulated silicon by means of horizontal and/or vertical movement of the conveying device, wherein the conveying device is completely encapsulated with respect to the outside and the forward movement of the granules is produced by a swaying movement of the conveying device by means of the excitation of at least one permanent magnet fitted to the conveying device by an electromagnetic field, wherein the electromagnetic field is applied to the encapsulated device from the outside.
The granular polysilicon has to be handled at various points in the production process. First of all, it has to be withdrawn from the reactor. The method described above is suitable for that purpose. Subsequently, it may have to be screened in order to separate it into different classes of particle sizes. For this purpose, it is transported by means of a transport container to the screening facility. Finally, the granules have to be packed. For this purpose too, it is customary to transport the granules by means of a container to the packing facility. Alternatively, the target material from the screening facility can also be collected in a stationary vessel. The vessel is connected directly to the packing facility.
As mentioned above, the granular polysilicon has bulk material properties. Therefore, experience from general bulk material technology is transferable to the granular polysilicon.
A problem in the handling of bulk materials is particle segregation.
Segregation by particle size arises when a central cone of bulk material forms in the middle in the course of filling of a vessel (or of a container or a silo). In the course of filling, the larger particles, because of their greater mass and hence higher kinetic energy, roll into the periphery (in the vessel wall direction), while the fine material accumulates principally in the center. The effect of such segregation over the cross section is that product streams with different particle size distribution are discharged successively as the material runs out.
If no measure is taken to counter particle segregation, an inhomogeneous production batch with respect to particle size is obtained in the production of bulk silicon materials in which the product is stored in vessels.
For many semiconductor and photovoltaics applications, however, maximum homogeneity of the particle size of the silicon raw material is required to assure stable processes.
Bulk material is generally stored in containers or silos. If bulk material flows out of a silo, a distinction is made between mass flow and funnel flow.
In the case of mass flow, the entire contents of the silo are in motion when bulk material is drawn off. Mass flow is only possible when the vessel walls are sufficiently steep and/or smooth. Moreover, what is called plug flow has to be achieved at the same time, in which all vertical silo cross sections also flow at the same speed. The main method of achieving this is a suitable design of the angle of funnel inclination.
Although it is extremely difficult, optimal dimensioning alone can achieve the desired backmixing.
One alternative is internal funnels, called binserts. They are smaller than the actual funnel and are placed in front of it. However, the use thereof is limited and the design thereof for use in the case of segregation is difficult.
If the funnel wall is too flat or too rough, funnel flow is established. In the case of funnel flow, at first only the bulk material in the region above the outflow orifice is in motion. Bulk material in the edge region of the silo is not discharged until the silo has been emptied completely. In the course of emptying, the bulk material in the center of the silo—i.e. the fines—is the first to be drawn off, while predominantly coarse material is discharged toward the end of the emptying. In the case of downstream packing of the bulk material, this would lead to different qualities in the individual packing units.
In a mass flow silo, in contrast, the bulk material which has been segregated in the course of filling merges in again, such that there is no trace of the segregation at the outflow orifice. Mass flow silos typically comprise conical or wedge-shaped funnels.
There have also been suggestions of counteracting particle segregation by moving the mixing vessel. However, the great technical complexity and high wall abrasion are disadvantageous. For granular polysilicon in particular, this approach is unusable since the wall abrasion leads to unwanted contamination of the high-purity silicon. Moreover, the movement of the bulk material can result in a post-comminution effect which gives rise, for example, to dust.
By altering the filling operation, it is possible to minimize segregation. It is possible to avoid large cones of bulk material by filling via several introduction points. This makes the situation somewhat less serious, but does not entirely prevent segregation. The complex filling system required additionally constitutes a contamination risk in the production of high-purity bulk Si materials.
Another possible approach to a solution is a discharge aid such as a controllable inner cone. The inner cone is mounted in the lower region of the vessel. This forms an annular gap between the cone and vessel wall, which simultaneously supplies the coarse material from the edge region and the fine material from the center of the vessel to the outlet, so as to result in a certain degree of backmixing.
A further measure for influencing the emptying operation involves what are called emptying tubes, which are equipped with gaps or holes. However, backmixing is only possible when emptying is sufficiently slow.
Usually, transport containers are used in the production of granular polysilicon to transport material from one manufacturing operation to the next.
The prior art has to date not offered any promising solutions for avoiding particle segregation in the handling of granular polysilicon. Transport containers with mass flow are impracticable since very high construction heights are required because of the steep angle of funnel inclination necessary. Since the center of gravity is then very high up, there is a risk that the transport container will tip over.